A conventional A/D convertor separately has an integration circuit for integrating a current signal and an A/D conversion circuit for performing A/D conversion on an integrated value obtained by the integration circuit, which has caused a problem of an increase in circuit scale. In order to suppress such an increase in circuit scale, there has been proposed an integration circuit provided with an A/D conversion function.
As the conventional integration circuit provided with the A/D conversion function, there has been proposed an integration circuit that compares an integrated value with a predetermined threshold while integrating a current signal, and discards the integrated value every time it reaches a threshold, to realize A/D conversion of the current signal based on the number of times the integrated value has reached the threshold.
However, in such an integration circuit, a delay in operation of a comparison circuit for comparing the integrated value and the threshold leads to a delay in discarding the integrated value or makes it impossible to integrate a current signal during a period for discarding the integrated value, and there has thus been a problem of deterioration in accuracy in A/D conversion. Further, in order to prevent ringing of an output at the time of discarding the integrated value and an erroneous operation of a comparator due to a slow response, a high-speed amplifier with a large phase margin is required, and there has thus been a problem of an increase in power consumption.